In general, a polarizing plate has a structure of laminating a triacetyl cellulose film (hereinbelow, referred to as TAC film) as a protection film on a polarizer by using a water-based adhesive composed of an aqueous solution of polyvinyl alcohol. However, both of the polyvinyl alcohol film used as the polarizer and the TAC film used as the protection film for a polarizer do not have sufficient heat resistance and humidity resistance. Thus, the polarizing plate including the above films has many drawbacks in terms of its use, since degree of polarization deteriorates, separation of the polarizer and the protection film occurs, or optical characteristics deteriorate under a high temperature or high humidity environment.
In addition, the TAC film has high variation in its in-plane retardation (Rin) and thickness retardation (Rth) according to environmental temperature/humidity, in particular, in the retardation for obliquely incident light. Upon employing a polarizing plate that includes the TAC film having such properties as a protection film in a liquid crystal display device, there are problems in that the view angle characteristic varies according to environmental temperature/humidity, and thus image quality deteriorates. In addition, the TAC film has poor dimensional stability as well as a relatively high photoelastic coefficient, depending on environmental temperature/humidity. Therefore, after a durability test under high temperature and high humidity environment, changes in the retardation characteristics occur locally, and thus image quality may deteriorate.
As an alternative to the TAC film, an acrylic resin is well known. However, the acrylic resin is easily broken or cracked, which causes a problem in transportation during production process of the polarizing plate, and reduces productivity.
In order to solve the problems, a method of blending the acrylic resin with other resins or a toughening agent (Japanese Patent Publication Nos. 2006-284881 and 2006-284882) or a method of co-extruding other resins (Japanese Patent Publication Nos. 2006-243681, 2006-215463, 2006-215465, and 2007-017555) has been suggested. However, there are problems in that these methods do not sufficiently provide the intrinsic properties of the acrylic resin, including high heat resistance and high transparency or have a complicated laminated structure.